The success of second generation (2G) mobile phones has motivated and enhanced the development of a wide range of wireless technologies, including for example 3G video phones, WiFi, WIMAX, ZigBee and Bluetooth. For cost effectiveness and space utilization wideband antennas that can accommodate several different communication systems are in high demand. In particular, antennas with unidirectional radiation patterns of various beam widths are of interest as they may be mounted on walls or vehicles without degrading their electrical characteristics and without affecting the aesthetics of the mounting bodies.
There are a number of techniques available for implementing wideband antennas with a unidirectional radiation pattern. The first approach is to put a dipole in front of a finite ground plane. However, since the size of the ground plane and the distance between the ground plane in terms of wavelength are frequency dependent, the antenna the disadvantages of large variations in gain and beam width over the operating band, different beam widths in the E and H planes, and strong radiation in the back side.
The second approach is through the use of patch antennas. The basic structure of a patch antenna is low in profile and has a unidirectional beam pattern, but is narrow in bandwidth. Several designs are now available to increase the bandwidth of patch antennas, such as U.S. Pat. No. 6,593,887 which describes a patch antenna with an L-shaped probe feed that is simple in structure and very wide in bandwidth. However, this class of patch antennas has the drawbacks of high cross-polarization, large variations in gain and beam width over the operating band, different beam widths in the E and H planes, and very strong radiation in the back side.
The third approach is to design an antenna consisting of a combination of an electric dipole and a magnetic dipole, commonly called a complementary antenna. It has been shown that if two complementary radiating elements of equal amplitude are combined properly in phase and orientation, the resulting radiation pattern will be equal in the E and H planes and the back lobe will be zero in strength, known as “cardioid” pattern shape. This concept has been realized in slot-and-dipole combinations where the slot is equivalent to a magnetic dipole in radiation, but the resulting designs are either narrow in bandwidth or bulky in structure.